This application relates to transmitting RF signals and, more particularly, to transmitting RF signals from a conductive enclosure, such as a parking meter.
Parking meters are an important source of revenue for many municipalities, but maintaining and monitoring the meters is a labor intensive process that consumes much of the revenue derived from the meters. Parking meters are also prone to vandalism and theft, which often leave the meters inoperable. In general, municipalities do not learn that parking meters are out of order, vandalized, or stolen until patrol or collection officers visit the meters. Enforcement of parking meter violations also requires a patrol officer to walk or drive within several feet of the meters so that the officer can read the meter displays. Many electronic parking meters include electronic processors that collect information about the meters, such as the amount of money stored in the meters and the time elapsed since the pay timers last expired.
This application describes wireless transmission circuitry that transmits information from conductive enclosures, such as parking meter housings. This circuitry allows remote monitoring of monetary collection units, such as parking meters and public telephones, that tend to attract vandals and thieves. Real-time alarms alert the owners of these collection units when someone attempts to tamper with the units. This circuitry also allows remote detection of parking meter violations. The circuitry can be made to fit within standard parking meter housings and to remain hidden from view.
In one aspect, the invention features a slot antenna for use in transmitting radio frequency (RF) signals. The antenna is formed on a circuit board from a layer of conductive material from which some material has been removed to form a slot. A signal feed element, such as a coaxial cable terminating at a coaxial connector, connects electrically to the layer of conductive material on one side of the slot, and a conductive element extends from the signal feed element across the slot to connect to the layer of conductive material.
In some embodiments, the conductive material surrounds the slot entirely. In other embodiments, the slot has at least one serrated edge. In many antennas, the length of the slot is often selected to equal xc2xd-wavelength at a selected transmission frequency, and the position of the feed element is selected to minimize return loss in the antenna.
In other embodiments, the antenna is shaped to fit within the space constraints of a particular type of enclosure, such as a parking meter. Some antennas are designed to replace the rate plates in conventional electronic parking meters and therefore include printed parking rate information on one surface.
In another aspect, the invention features an electronic parking meter having processing circuitry that gathers data about the parking meter and RF transmitting circuitry that modulates the data onto an RF carrier wave to form an RF signal. An RF antenna in the meter then transmits the RF signal. A housing, typically formed from a conductive metal, encloses the processing circuitry, the RF transmitting circuitry, and the RF antenna.
In another aspect, the invention features a parking meter rate plate, one surface of which includes printed parking rate information, and another surface of which includes an RF antenna. In some embodiments, the antenna is a slot antenna formed on a printed circuit board.
In yet another aspect, the invention features a technique for use in transmitting data from an electronic parking meter. The technique involves reading data from the meter electronically, modulating the data onto an RF carrier wave to form an RF signal, and delivering the RF signal to an RF antenna located within the parking meter.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.